Biomarkers are useful measurements to monitor ranges of neurological and biochemical activity. They can be used as warning signs of poor adaptation to changes in either internal or external environments. The eye is an apt structure to use for obtaining biomarkers, since it interacts with multiple systems. Light affects both chemical and neurological function in both the central and the autonomic nervous systems, and effects can be measured by eye movement changes, body changes (breathing, heart rate, etc), electrophysiological changes (brainwave testing EEG) by chemical assays (of blood, tears). For example, pupil size and response during visual scanning tasks has been touted as a potential biomarker for autism (Martineau, J., N. Hernandez, et al. (2011). “Can pupil size and pupil responses during visual scanning contribute to the diagnosis of autism spectrum disorder in children?” J Psychiatr Res 45(8): 1077-1082), the osmolarity in human corneal tear layer is though to possibly be a biomarker for dry eye severity (Suzuki, M., M. L. Massingale, et al. (2010). “Tear osmolarity as a biomarker for dry eye disease severity.” Invest Ophthalmol Vis Sci 51(9): 4557-4561) and disruptions in rapid eye movement during sleep is found to correlate with amounts of stress (Mellman, T. A., V. Bustamante, et al. (2002). “REM sleep and the early development of posttraumatic stress disorder.” Am J Psychiatry 159(10): 1696-1701).
Until 2003, it was commonly thought that the eyes had only two photoreceptors (cones and rods) and that its only purpose was to see. In 2003, a discovery was made of a third photoreceptor in the ganglion layer of the retina (rather than the photoreceptor layer). Approximately 2% of retinal ganglion cells contain melanopsin and are sensitive to changes in light. However, those cells, termed ipRGC—intrinsically photosensitive retinal ganglion cells, are not image producing cells as are the cones and rods. Such cells primary role is believed to signal light for unconscious visual reflexes, such as pupillary constriction and regulating a number of daily behavioral and physiological rhythms, collectively called circadian rhythms.